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Sometimes it is just not practical to "open fire on everyone" Ammo conservation, and biggest threat targets have everything to do with it.

If the objective of the sisters landing was to ensure that all of the tyranids were slaughtered, do you think they would kill some DE that were killing Tyranids?

They would keep an eye on them, but would not actively kill them until the Tyranids were dead, or the DE started shooting at the sisters.

It is called tactics, not simple ignorance, and the Tyranid Hive mind has a different set of tactics "Eat Everything".